چكيده لاتين :
Introduction
According to Glantz,s theory, desertification term has about one hundred definitions(Glantz, 1977) . The common point in all these definitions is the severe demolition of the environment due to desertification..Some of these definitions are: ecosystem degradation (Reining, 1978), degradation of plant species (Le Houerou, 1975), decrease of ecosystem production and a decrease in biological production. Each of these definitions shows the ecosystem change from favorable conditions to unfavorable conditions and decrease of biological production. In the Environment and Development conference of the United Nations held in Riodojaniro in 1992, desertification was defined as: land degradation and demolition of dry , semi dry and dry semi humid areas as the result of human activities and climatic changes. At present desertification is a serious problem plagued many countries of the world. Considering and paying attention to desertification for a country like Iran which 80 million hectares of which is covered by dry and semi dry areas, is a necessary and inevitable issue.12 million hectares of this area is covered by running sands , 6 million of which is formed of active sand hills (Refahi , 2006) . Wind erosion due to wind blowing usually happens on bare areas with no surface vegetation cover. Wind is said to horizontal movements of the atmosphere. Wind is subject to pressure, therefore changes of pressure produces different winds in the atmosphere. At the low levels of the atmosphere, molecules are near to each other, the contacts are great and the pressure is high, therefore the wind blows in the direction which has the highest pressure (Moghimi 2006). Soil erosion is a phenomenon, in which soil is displaced due to environmental factors like water, wind, gravity force and so on and after carrying, is deposited in another area. The under study area is considered as one of the driest areas in the world and wind erosion is actively salient in this area and each year, makes a great deal of soil out of access. In the areas with no control on erosion, the soils are gradually eroded and lose their fertility. Erosion, not only weakens the soil and desolates the farms and makes great and irreparable damages, but also through sedimentation of materials in water ways, dam reservoirs, ports and decreasing their impounding capacity, makes Rstages. Such situation is visible in the study area which is a large part of Sistan plain and Hamoon Lake. Wind erosion is saliently active in this region. During wind erosion, the particles are moved by three methods of creeping, jumping and suspension. Particles mostly move by jumping movements. Diameters of jumping particles are usually between 0.05 up 0.5mm. Jumping movements of particles are usually happened at the limit of 0.1up to 0.15mm (Refahi 2006).increase of soil erosion, in addition to the soil features and nature depends greatly on the jumping particles. So that the particles which are moved by jumping method, when contacting with the ground surface, move the resting particles of the ground surface. During this process, the particles which their threshold speed is more than the wind rate will move.
Research Methodology
In general, the effective factors on intensifying the wind erosion in the area under study can be divided in to two human and environmental factors. Environmental factors are mainly rooted in the structure of climatic changes, geology, pedology and geomorphology, and human factors (management) can be used for uncontrolled grazing of livestock, clearing plants, changing forest and pasture lands to agricultural lands and non controlled and inappropriate use of lands. Although for specifying the priorities , it is sometimes required to use the statistics and comparing the under study parameters in particular time periods , but until achieving this methods , it is possible to use the information, experiences and expressions of the people who have obtained during the long times of living in this area.With respect to the above said items, in this research for estimating the severity of desertification, IRIFR.E model, presented by the Iranian researches ( Ekhtesassi and Ahmadi) has been used. IRIFR model like PSIAC model, evaluates the role and effect of nine factors in wind erosion and the resulting sedimentation amount. A score is given to each factor depending on its severity and weakness and its effect on desertification. By adding up the above scores, degradation severity of geomorphologic facies, whish is the basis of studies for soil erosion, will be estimated. The following table shows the nine factors effective on intensifying the wind erosion(able 1-)
Table-1: Effective factors on soil erosion and sediment generation based on IRIFR model
No. AEffective factors on soil erosion and sediment generation based on IRIFR model Score
1 Litlogy 0-10
2 Geomorphology 0-10
3 Wind speed and condition 0-20
4 Soil and land cover -5-15
5 Vegetation cover density -5-15
6 Surface erosion effects 0-20
7 Soil moisture 0-10
8 Type and dispersion of wind sedimentations 0-10
9 Land use planning and management -5-15
Discussion and Results
With respect to the field studies, the land use map was designed which includes nine facies as the following figure ( figure No. 10). Specifications of geomorphologic facies has been presented in the following table (table No. 12).
Fig-1: working units maps obtained from the study area using aerial photo processing,
Topographic and geology maps and the visits.
Table-12 Units,types and Geomorphologic facies types were identified in the study area
Name and code of Geomorphologic Unit Name and code of Geomorphologic type Name and code of Geomorphologic Facies Symbol
2 Covered plain 3-2 Covered plain 1-3-2 Obsolete undesirable agricultural lands R2.1
2-3-2 Bare land R2.2
3 Playa 1-3 Clay plain 1-1-3 Salty and bloated land R3.2
2-1-3 dry earth crust without vegetation F1.1
3-1-3 dry earth crust with cane F1.2
4-1-3 Hard and dry earth crust with Tamarix tree F1.3
2-3 desert 1-2-3 Humid region L1.1
2-2-3 Kavir Lake L2.1
3-2-3 Lurg L3.1
In continue, after specifying the homogenous units (land use map), the considered factors in each unit has been evaluated based on the designed table and the results obtained from evaluation and scoring the under study indices in this study have been presented in the following table(table 13) . Finally with respect to the obtained results from pervious stages, the map for current condition of desertification is being obtained by using IRIFR model in the under study area and based on the following figure (figure No.2)
Conclusion
The results of this research indicate this fact that desertification has been occurred actively in the area and has an increasing trend and also show that the under study area is placed at desertification class of medium, severe and very severe and from all of the under study area, 26% equal 22791 hectares is placed in medium desertification class, 44.3% equal to 38832.6 hectares in severe desertification class and 29.7% equal to 26034.4 hectares in very severe desertification class . Regarding the obtained results from this research and performed visits of the area, it is possible to prevent the progress of desertification process in the areas which are placed in low critical places through performing biological and mechanical plans, otherwise, due to special conditions dominant on the area, the condition of these areas becomes more critical and severe. The areas of severe class with an area of more than 38832.6 hectares, have dedicated itself most part of the area which includes humid area, Kavir lake , dry and hard earth crust without any vegetation cover together with cane. In case of lack of control, this phenomenon is transferred to other areas and makes the situation of the area more critical. After providing the land sensitivity to wind erosion map, each class limit in all the area is calculated by using digit system. The calculation and relative percentage of wind erosion classes in the study area has been presented in the following table.
Table 14. Percentage of the area related to each level of desertification class in the study area
IRIFR score Limit Condition Erosion Intensity classes Region Area in Hectares Area Percentage
0-25 Non-sensitive I - -
25-50 few II - -
50-75 Medium III 22791 26
75-100 Severe IV 38832.6 44.3
100 > Very Severe V 26034.4 29.7
Estimation of sediment yield resulting from wind erosion in the study area by using IRIFR model:
To evaluate sediment yield potential of wind erosion in the study area by using IRIFR model, the relation presented between sedimentation degree and amount of sedimentation production as below, was used:
QS = 41[EXP (0.05R)]
In which:
Qs is annual sediment yield, tone per Square kilometer
R is Sediment yield degree (using total score factors affecting wind erosion)
For obtaining the R sedimentation degree, in each of geomorphologic facies and all of the study area from the total score, the effective factors on wind erosion is extracted and by using the above relation, the sedimentation potential resulting from wind erosion has been estimated. Results of evaluating sedimentation amount of the area has been presented in the following table.
Table No.15: results of evaluating sedimentation amount at the study area
Geomorphological Facies Sediment
Degree Specific sediment
Ton/Km2/year Specific sediment
Ton/hec/year
R2.1 92.4 25.4161 41.6
R2.2 98.5 5645.26 56.5
R3.2 99 5788017 57.9
F1.1 105.6 8051.16 80.5
F1.2 74.5 1700.33 17
F1.3 71.5 1463.47 14.6
L1.1 86 3021.7 30.2
L2.1 98 5505.9 55
L3.1 99.1 5817.18 58.2
Average 91.62 4002.1 40
Keywords: Desertification, Wind erosion, IRIFR, Sistan plain.
